The present invention is related to riveting and in particular to forming riveted pivot joints including a desired amount of clearance.
It is common in manufacturing to want a joint in which a rivet serves the dual purposes of both fixing two or more parts together and acting as a pivotal shaft, as in pliers joints, scissors joints, wire cutters, or various types of pinions. Rivet tension or clearance in such a joint is a factor in determining the amount of friction between two or more pivotally interconnected members. In a joint as in a tool such as pliers, it is usually desired to have two or more pivotal members in contact with one another, but not held so tightly together that friction interferes with their use, nor with so much clearance that the two parts of a tool feel loose or sloppy with respect to each other. In the case of scissors or wire cutters, such looseness may detract from the effectiveness of the tool in its primary cutting function. Such a tool with a loose or sloppy rivet joint is commonly perceived as having low quality.
In the past it has been difficult to rivet two parts such as pliers jaws or scissors together with the use of automatic machinery, and final adjustment of such joints has had to be done manually by skilled personnel. Some amount of success has been obtained by using shouldered rivets and then using accurately controlled time and pressure to form a rivet head. In order for controlling time and riveting pressure to be successful, the hardness of rivets must be accurately controlled, and as little pressure as possible must be used, in order to minimize the clamping pressure exerted by the tool forming rivet head. Unless the parts being connected and the rivets being used are produced to very close tolerances, however, these methods have less than completely satisfactory results, and it is therefore expensive to make such rivet joints.
The most widely used method of controlling the amount of tension or clearance in rivet joints, particularly in tools whose parts pivot with respect to each other, is manual adjustment. Manual adjustment means that after a rivet joint has been formed by machinery, hand tools are used to tighten or loosen the joint as necessary. This often results in inconsistent quality of pivot joints or imperfections in the appearance of a rivet head.
One known method of assembling pliers is disclosed in Thomson U.S. Pat. No. 1,177,738, which teaches use of a spacer of fibrous material interposed between the bearing surfaces while a rivet is formed, and later removal of the spacer to provide the desired amount of clearance between bearing surfaces. This method has not found great acceptance in industry, perhaps because of the difficulty of removing the spacer from between the jaws of tools made using the method.
Christensen U.S. Pat. No. 3,747,194 discloses the use of a preloading clamping pressure to hold together the parts being fastened, before the formation of a rivet head. While this provides reliably tight rivet joints, it is not apparently intended to produce rivet joints including clearance to permit connected parts to pivot.
What is needed, therefore, is an improved method and apparatus for forming rivet joints having a very small, but accurately established, amount of clearance between the parts riveted together, so that the parts are pivotally movable with respect to one another, with neither excessive friction nor excessive clearance, and without the need for manual adjustment.